As a power source of a mobile device, or the like, a lithium ion secondary battery is mainly used. The function of the mobile device or the like is diversified, resulting in increasing in power consumption thereof. Therefore, a lithium ion secondary battery is required to have an increased battery capacity and, simultaneously, to have an enhanced charge/discharge cycle characteristic.
Further, there is an increasing demand for a secondary battery with a high output and a large capacity for electric tools such as an electric drill and a hybrid automobile. In this field, conventionally, a lead secondary battery, a nickel-cadmium secondary battery, and a nickel-hydrogen secondary battery are mainly used. A small and light lithium ion secondary battery with high energy density is highly expected, and there is a demand for a lithium ion secondary battery excellent in large current load characteristics.
In particular, in applications for automobiles, such as battery electric vehicles (BEV) and hybrid electric vehicles (HEV), a long-term cycle characteristic over 10 years and a large current load characteristic for driving a high-power motor are mainly required, and a high volume energy density is also required for extending a driving range (distance), which are severe as compared to mobile applications.
In the lithium ion secondary battery, generally, a lithium salt, such as lithium cobaltate, is used as a positive electrode active material, and a carbonaceous material, such as graphite, is used as a negative electrode active material.
Graphite is classified into natural graphite and artificial graphite.
Among those, natural graphite is available at a low cost. However, as natural graphite has a scale shape, if natural graphite is formed into a paste together with a binder and applied to a current collector, natural graphite is aligned in one direction. When an electrode made of such a material is charged, the electrode expands only in one direction, which degrades the performance of the electrode. Natural graphite, which has been granulated and formed into a spherical shape, is proposed, however, the resulting spherical natural graphite is aligned because of being crushed by pressing in the course of electrode production. Further, the surface of the natural graphite has high reaction activity, resulting in a low initial charge-discharge efficiency and poor cycle characteristics. In order to solve those problems, Patent Document 1 and the like propose a method involving coating carbon on the surface of the natural graphite processed into a spherical shape. However, sufficient cycle characteristics have not been attained.
Regarding artificial graphite, there is exemplified a mesocarbon microsphere-graphitized article described in Patent Document 2 and the like. However, the article has a lower discharge capacity compared to a scale-like graphite and had a limited range of application.
Artificial graphite typified by graphitized articles made of oil, coal pitch, coke and the like is available at a relatively low cost. However, although a crystalline needle-shaped coke shows a high discharge capacity, it tends to align in a scale shape and be oriented in an electrode. In order to solve this problem, the method described in Patent Document 3 and the like yield results.
Further, negative electrode materials using so-called hard carbon and amorphous carbon described in Patent Document 4 are excellent in a characteristic with respect to a large current and also have a relatively satisfactory cycle characteristic.
Patent Document 5 discloses artificial graphite being excellent in cycle characteristics.
Patent Document 6 discloses an artificial graphite negative electrode produced from needle-shaped green coke.
Patent Document 7 discloses an artificial graphite negative electrode produced from cokes coated with petroleum pitch in a liquid phase.